Deserts and desert-like regions occupy 1/4 to 1/3 of the terrestrial world and are expanding. Reasons for this desertification include using irrigation water with a high salt concentration, dams, irrigation and deficient draining equipment, and sea water reaching farm land, all causing salts such as NaCl, Na.sub.2 SO.sub.4, MgCl.sub.2, CaCl.sub.2 and the like to accumulate in soil. In such salt-containing soil, almost all plants (excluding specific salt-tolerant plants) experience strongly restrained growth or lesions. According to Szabolcs (in the United Nations Desertification Prevention Congress at Nairobi, 1977), the soil accumulated with salts in the world had reached 952 million ha in 1977. To check enlargement of the deserts, afforestation and plant rearing have been attempted in the areas of the high-salt soil. However, because few plants can grow in the high-salt soil, the attempts to date are insufficient. Furthermore, because crop productivity in the high-salt soil is markedly low, little of the high-salt land is used as farmland.
In the Middle East, salt water processed into fresh water is used for agricultural irrigation. However, because enormous processing of salt water into fresh water requires much energy and much expense, and has much environmental impact, obtaining large quantities of irrigation water with a low salt concentration at low cost has been very difficult.
Moreover, in advanced nations, fertilization causes salt accumulation with corresponding problems such as deteriorated crop yield.
Some aspects of selection and rearing of salt-tolerant plants have been studied. For example, a salt-tolerant variety of rice is known. Furthermore, breeding salt-tolerant plants by adapting a callus of a plant to a high salt medium and reproducing the plant has been attempted. Moreover, a search for a gene related to salt-tolerance and a recombination of the gene has been studied to improve plant salt-tolerance.
On the other hand, there have been few studies directed to developing an agent for improving salt-tolerance. Only use of gibberellin (GA.sub.3) has been reported. (Zhao Ke-fu et al., Aust. J. Plant Physiol., vol. 13, pp. 547-551 (1986)).
However, in these above-mentioned earlier methods for improving plant salt-tolerance, the salt-tolerance obtained by a selection or bleeding is insufficient, and what characteristic of plants relates to salt-tolerance was unknown.
In addition, in the study of plant callus, the desired plant could not be obtained, and in the gene recombination, the gene related to salt-tolerance was not specified. Further, in developing an agent for improving salt-tolerance, prior to the invention of this application, others only knew merely that gibberellin slightly relieves salt stress.
On the other hand, 5-aminolevulinic acid and salts thereof are useful as a herbicide (U.S. Pat. No. 5,127,938), an insecticide (EP-A-326835), to accelerate plant growth (U.S. Pat. No. 5,298,482) and an agent for improving the pigmentation of apple rinds (U.S. Pat. No. 5,318,788). Parts of esters of 5-aminolevulinic acids and N-acyl-5-aminolevulinic acids are useful as a herbicide (JP-A-4-9360; the term "JP-A" as used herein means an unexamined published Japanese patent application). However, it was not known previously that 5-aminolevulinic acid and derivatives thereof improve plant salt-tolerance.
Developing an effective method for improving plant salt-tolerance would prevent the desertification of arable land and would allow crop production in high-salt soil. Correspondingly, an important food problem associated with ever increasing population would be solved or alleviated. Furthermore, an effective method of achieving plant salt-tolerance would have many other benefits, such as improving salt-to-fresh water distillation for irrigation water and reducing the amount of irrigation water needed.